The greater the loss in performance of an oxygen probe, the slower is the reaction thereof to changes in the oxygen content of the exhaust gas which flows around the probe. In this way, the period duration of the two-level control operation is increased which takes place with the aid of the probe signal. The controller output signal is generated with the aid of an integration process during the controller oscillations. For this reason, the amplitude of the controller oscillations also increase with an increase in the oscillation period. The probe is deemed to have a performance loss which is impermissibly great when the measured period duration exceeds a threshold value.
If probes are mounted forward and rearward of a catalytic converter for the lambda control, then the signal of the rearward probe, which always has a lower performance loss than the forward probe, is utilized to compensate for signal changes which are caused by the performance loss of the forward probe. When the compensation variable exceeds a threshold value, this shows that the forward probe has undergone a performance loss which is impermissible. The arrangement for the method just described utilizes the signal of a rearward probe wherein the loss of performance is low and operates with greater precision than the first-described arrangement which monitors the controller period duration. However, it is a disadvantage that the second, rearward probe is absolutely necessary.